LUNGS. 263 



good fliers (e.g. Larus, Sterna) the bones are not pneumatic. 1 In 

 these cases, however, a compensation is effected by a more marked 

 development of the muscles, and the abdominal (posterior) air-sac, 

 which in no Birds appears to be entirely wanting, is here well 

 developed. 



The air-sacs must be looked upon as integral parts of the respiratory 

 apparatus : a greater amount of air can, by their means, pass in and out during 

 inspiration and expiration, especially through the larger bronchi, and con- 

 sequently there is less necessity for the expansion of the lung-parenchyma. 

 The function of the prolongations of the air-sacs lying towards the outer surface 

 of the body consists in the giving off of watery vapour and in regulating the 

 heat of the body. Those which extend in between the muscles, and supplant 

 the connective and fatty tissue in these regions, have a further importance in 

 causing less power to be lost in friction. 



But by far the greatest importance of the air-sacs lying towards the 

 periphery consists in the enlargement of the anterior thoracic region, princi- 

 pally that surrounded by the pectoral arch. A larger development of the 

 skeleton can thus take place, giving an increase of surface for muscular 

 attachment without any considerable increase in weight. Everything, in fact, 

 combines to establish an organ of flight with a large wing-surface and an 

 increased strength of the muscles. 



Mammals. As already mentioned, main and lateral 

 bronchi can be distinguished in Mammals. The pulmonary artery 

 crosses the main bronchus at its anterior end, and this point may 

 be taken as dividing the lateral bronchi into two systems an 

 eparterial lying anterior (above) and a hyparterial lying 

 posterior to (below) the artery. 



The hyparterial series is always well developed, and consists of 

 a double row 2 of lateral bronchi ; the eparterial system, on the 

 other hand, gradually becomes of much less importance, and in 

 certain cases is represented only by a single lateral bronchus on 

 either side (Fig. 211, c, b, a), and, as a rule, even the left of these 

 disappears, only the right remaining. This eparterial bronchus, 

 whether developed on one or on both sides, may change its position 

 on the main bronchus so as to arise from the trachea. 3 



In by far the greater number of Mammals then, the left epar- 

 terial bronchus has disappeared, while the right is retained ; this 



1 The pneumaticity of the bones is not a special peculiarity of Birds : amongst 

 Mammals, frontal, maxillary, and sphenoidal sinuses are present in Anthropoids, 

 Elephants, and Marsupials for instance ; the skull of Crocodiles is also strongly 

 pneumatic. All these sinuses communicate with one another, and also with the tym- 

 panic cavity. They are in many cases developed in order to give a greater surface for 

 the attachment of muscles, and also to effect a saving of material and a lightening of 

 the skull. 



2 The pulmonary artery passes backwards between the roots of the hyparterial 

 bronchi, while the corresponding vein runs along the ventral side of the main 

 bronchus (Fig. 211, A, V). 



3 The left eparterial bronchus may also disappear (Hystrix), and thus the ter- 

 minal stage of a process begun in Birds is reached. It is difficult to find an explana- 

 tion for these facts ; they may possibly have something to do with the gradual 

 disappearance of the cervical and lumbar ribs, in connection with the shortening of 

 the thorax (comp. p. 51). 



